1. Field of the Invention
The present invention relates to the cleaning of semiconductor wafers. More particularly, the present invention relates to a method of and apparatus for washing, rinsing and drying semiconductor wafers all within in one cleaning chamber.
2. Description of the Related Art
Semiconductor devices are generally manufactured by selectively and repetitively performing respective unit processes such as photolithography, etching, ashing, diffusion, chemical vapor deposition, ion implantation and metal deposition processes. These processes, when performed in series, produce at least one or more conductive layers, semiconductor layers and insulator layers on a wafer. Furthermore, each unit process is typically followed by a respective cleaning process for removing impurities from the wafer, e.g., a layer of undesirable material, byproducts of the reaction created during the unit process, or various kinds of foreign substances.
A prior art semiconductor wafer cleaning system for removing various impurities from a wafer will now be described with reference to FIGS. 1 and 2. A given number of wafers W are mounted in a cassette C after a unit process has been performed on the wafers W. The cassette of wafers W is then transferred by a loading unit 10 to an aligning unit 12 of the cleaning system (ST102). Next, the wafers W are aligned by the aligning unit 12 (ST104) so as to all be oriented in the same direction. The wafers W are next transferred by a transfer device 14 (ST106) to a first transfer unit 16. The first transfer unit 16 checks the number of wafers W in the cassette C (ST108), removes the wafers W from the cassette (ST110), and transfers the wafers W to a robot 18 (ST112).
The wafers W are conveyed by the robot 18 sequentially through a plurality of washing tubs 22 (ST114). At least one first washing tub 22 contains a cleaning solution of an acid or alkaline diluted with de-ionized water for cleaning the wafers W, whereas at least one second washing tub 22 disposed immediately downstream of the first washing tub(s) 22 contains de-ionized water to rinse away any of the chemicals (acid or alkaline) remaining on the surface of the wafers W. The cleaning process that is carried out in each respective washing tub 22 is facilitated by causing the cleaning solution and/or the de-ionized water to overflow the tub 22. Also, the final step of the rinsing procedure is a non-resistance measurement of the impurities.
A drying unit 24 is disposed adjacent to the washing tubs 22 to remove the de-ionized water from the wafer W (ST116). The drying process is performed in an atmosphere of vapors of Isopropyl Alcohol (IPA) to remove moisture from the surface of the wafers W.
The dried wafers W are then transferred to a second transfer unit 26 by the robot 18 (ST118). The second transfer unit 26 aligns the wafers W (ST120) and simultaneously checks whether the same number of wafers W counted by the first transfer unit 16 have been received, i.e., whether all of the wafers have been cleaned (ST122). Furthermore, the second transfer unit 26 mounts the wafers W into a cassette C positioned at a stand-by unit 20 (ST124). Once all of the cleaned and dried wafers W are mounted in the cassette C, the cassette C is unloaded from the cleaning system (ST126).
As is clear from the description above, the components of the prior art cleaning system are disposed in line so that the various steps of the overall cleaning procedure can be carried out in succession. More specifically, the wafers W are transferred from a washing tub(s) 22 containing chemicals to a washing tub 22 containing only de-ionized water (for a primary rinse), and then again to another washing tub 22 containing de-ionized water (for a secondary rinse). The rinsed wafers W are then moved from the last washing tub 22 to the drying unit 24.
The wafers W are thus exposed to the air for a considerable period time while the wafers W are being moved to the drying unit 24. The exposure of the wafers W allows oxygen O2 in the air to dissolve into moisture on the wafer surface. Spots of SiOx are then formed on the wafer surface because the oxygen O2 reacts with the poly Si of the wafer and then dries naturally. These spots remain as they are as inorganic matter of the silica group or adsorb various other foreign substances in the air. These so-called water spots can produce contact defects.
Furthermore, the various chemicals or de-ionized water in which the wafers W are submerged are kept flowing from a lower part to an upper part of the washing tub 22 (overflow cleaning method). Thus, a difference in the etching rate of the wafers occurs, as between the lower and upper portions of the wafers 22 in the tubs 22.
FIG. 3 illustrates a conventional wafer cleaning apparatus aimed at obviating the problems described above in connection with the system of FIG. 1. In this apparatus, a plurality of wafers W are separated from the cassette C and stand by, a cleaning chamber 30 is filled with a cleaning solution comprising chemicals diluted with the de-ionized water, and the wafers W are submerged in the cleaning solution. After a given time elapses, i.e., once enough time has been given for the cleaning solution to clean the wafers W, the cleaning solution is discharged from the cleaning chamber 30 while de-ionized water is supplied into the chamber 30. Thus, the wafers W are rinsed in the same chamber 30. Next, the de-ionized water is drained from the chamber 30 while the chamber 30 is filled with IPA vapor, thereby drying the wafers. Subsequently, heated N2 gas is introduced into the cleaning chamber 30 while the IPA vapors are discharged.
Such a cleaning system washes, rinses and dries the wafers W all within in one chamber 30 to minimize the exposure of the wafers W to the air during the overall cleaning process. However, this process requires much more time than the process of FIG. 2 based on an in-line disposition of washing tubs 22.